Excess-Phase Response
Canonical Definition
Excess phase is the phase remaining after subtracting the minimum-phase response associated with the measured magnitude, with any explicitly handled bulk delay treated consistently. Excess group delay is the frequency derivative of that excess phase. These quantities reveal delay, all-pass, reflection, summation, and other non-minimum-phase behavior under the stated measurement conditions.
What The Trace Can Show
- a constant channel or transport delay
- frequency-shaped all-pass behavior
- driver or crossover misalignment
- reflection and multipath interference
- two-source cancellation and summation
- measurement time-zero, clock, window, or noise problems
The trace identifies behavior that needs investigation; it does not identify the cause by itself.
Phase Versus Group Delay
Excess phase is an angular difference versus frequency. Excess group delay is the negative slope of that unwrapped phase. A straight phase slope can represent a constant time delay. A shaped slope produces frequency-dependent excess delay.
Do not interpret phase or group delay where level is near the noise floor, inside a severe null, or outside the valid measurement window.
Measurement Workflow
- Use a shared and repeatable timing reference.
- Document any time-zero shift or removed bulk delay.
- Measure a wide enough frequency span with valid signal-to-noise ratio.
- Set and record impulse windows and calibration handling.
- Generate a new minimum-phase reference after window changes.
- Compare individual sources, their sum, and nearby positions.
Correction Choices
| Finding | Possible action | Do not assume |
|---|---|---|
| Constant excess delay | channel delay or time-reference correction | that the entire band needs FIR inversion |
| Localized all-pass-like rotation | delay, all-pass, crossover, or constrained phase correction | that magnitude EQ will solve it |
| Position-sensitive cancellation | placement, source balance, crossover, polarity, or spatial compromise | that one-point inverse filtering will generalize |
| Unstable trace near a null | improve measurement and diagnose contributors | that a large boost is safe or useful |